The present disclosure relates to optical waveguides, and more particularly, to coatings for optical waveguides.
An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. An optical waveguide is a basic element for confinement and transmission of light over various distances. Optical waveguides are used in a variety of applications, including integrated optical circuits and long distance light wave communication. One type of optical waveguide includes a solid body that transmits light, such as a laser beam, and confines the light beam to a specific region of the solid body. A planar waveguide, also known as a slab waveguide, confines the light beam to a planar region within the waveguide, thereby guiding light in only one dimension.
Some solid state optical waveguides include a core that confines light and a cladding or substrate that surrounds the core. A guided light wave propagates in the waveguide along the longitudinal direction. For example, a conventional slab waveguide may include three layers of materials. When light is injected from the side into the middle layer, light may be confined in the middle layer by total internal reflection (TIR). TIR occurs when the refractive index of the middle layer is larger than the surrounding layers.
A waveguide that has an index profile with abrupt changes between the core and cladding is called a step-index waveguide. A waveguide with an index profile that varies gradually is a graded-index waveguide. When a waveguide with an active laser ion doped into its core is used in combination with a pumping laser, an optical signal propagating in the core of the planar waveguide may be amplified. The pumping laser creates a population inversion in the active ions which enables amplification by stimulated emission of radiation as the signal passes through the waveguide. In a cladding pumped waveguide where pump light fills both the core and one or more inner cladding layers adjacent to the core, additional outer cladding layers with an even lower refractive index than the inner cladding layers may be used to provide TIR for the pump light at the inner cladding/outer cladding boundary. Beyond the core/cladding structure used to guide the pump and signal energy, within the waveguide, one or more additional layers of material may be added to the waveguide to control stray light and/or absorb amplified spontaneous emission and/or suppress parasitic oscillations. The additional layers should possess several characteristics, including the ability to absorb light and/or to reflect light back into the core and/or cladding(s). In addition, the additional layers should possess desirable mechanical properties that enable a robust interface between the waveguide and its mounting/cooling structure.